The fact that the ostrich-like dinosaurs did not develop feathers until adulthood suggests that they were only used once they were sexually mature, perhaps for courtship rituals, to attract partners or for brooding.

The wing-like forelimbs and feathers most likely developed further for other purposes such as flying or keeping warm at a later stage of evolution, researchers wrote in the Science journal.

Dr Darla Zelenitsky, who led the study, said: "This dinosaur was covered in down-like feathers throughout life, but only older individuals developed larger feathers on the arms, forming wing-like structures.

"This pattern differs from that seen in birds, where the wings generally develop very young, soon after hatching."

Co-author François Therrien, of the Royal Tyrrell Museum of Palaeontology, added: "The fact that wing-like forelimbs developed in more mature individuals suggests they were used only later in life, perhaps associated with reproductive behaviors like display or egg brooding."

The fossils marked the first discovery of ornithomimid specimens preserved with feathers, and suggest that all dinosaurs of the same family, would have been covered in feathers like many other types of dinosaur.

They were also the first of their kind to be found in the Western Hemisphere, with most other feathered dinosaur skeletons limited to sites in China and Germany.

While previous evidence of feathers had only been found in fine-grained rock, the new fossils were unearther in sandstone, suggesting that many more may be found in rocks deposited by ancient rivers across the world.